1. Can Components and Construction
The conventional two-piece can includes two principal components, namely a can body and a lid (or top end). The can body includes a very thin, generally cylindrical, side wall and a thin, generally upwardly extending domed bottom formed integrally with the side wall at one end of the side wall. The opposite end of the can body side wall is joined to the separately formed top, typically with a double seam, but only after a carbonated beverage or other gas-charged/gas producing product has been introduced into the internal cavity provided by the can body. Can bodies are typically constructed using an aluminum alloy or, less frequently, steel or other materials, and are normally fabricated by a drawing and ironing operation.
In the drawing and ironing operation, a plurality of circular blanks of metal are initially punched from a thin metal sheet stock. Each blank is then drawn into the form of a relatively shallow cup. Next, in a sequence of ironing operations, the cup is placed over the end of a punch and forced through a set of dies which stretch and thin the side wall significantly until the cup becomes a can body having a desired height. However, the bottom of the can retains essentially the original thickness of the sheet stock even after the side wall is ironed. In the last ironing step, the punch also presses the bottom of the can body against an end-forming die to impart a generally upwardly extending domed configuration to it, i.e., such that the center of the bottom of the can body extends toward the interior of the can body further than the periphery of the bottom of the can body. After ironing, the top portion of the side wall is trimmed to ensure a flat top edge. The finished can bodies go through a number of additional operations, e.g., washing, decorating, curing, necking, and inspection, before being filled and then sealed with a lid.
2. Design Considerations
The design of a two-piece can must address and balance three often conflicting factors. First, the can must withstand physical forces--both internal forces arising from the pressure of the can's contents, and external forces experienced at different points in the can's service life. Failure to withstand physical forces results in obvious defects, such as punctures, which allow the contents of the can to escape or spoil, but it also results in undesirable physical deformation of the can, such as pressure-induced buckling of the lid (lid failure) or partial or total reversal of the domed bottom, which cause the can to be unsalable. Second, the can must require as little material as possible in its construction. Two-piece metal beverage cans are currently produced in quantities exceeding 90 billion cans per year in the United States; therefore, even a small reduction in the material required for each can produces significant economic benefits. Finally, a can design must have external characteristics that are compatible with the equipment and environmental conditions encountered during all phases of its life cycle, including production, filling and seaming, packaging, transportation, retailing, stacking and consumer use.
A typical balancing issue arises when the necessary maximum allowable pressure of a can conflicts with attempts to reduce the amount of material used in the construction of the can. The maximum allowable pressure of the can is the maximum internal pressure it can withstand without suffering excessive deformation or pressure-induced failure. To withstand internal pressures arising from a typical commercial volume, or "fill," of carbonated beverage at maximum values of the generally accepted ranges of carbonation, temperature (e.g., during heat pasteurization of beer), or physical agitation (e.g., rough shipping or handling), conventional cans currently require a maximum allowable pressure of 90 to 95 psig. "Lightweighting" refers to design modifications which decrease the overall amount of aluminum or other material used in the can, often by redesigning the lid or bottom profile or using thinner sheet stock for one or both of the two components. These efforts may result in reduced resistance of the lid or the concave domed bottom, to undesirable deformation. Thus, in general, lightweighting efforts must stop when they reduce the strength of the can below the necessary maximum allowable pressure.
To allow further lightweighting efforts, some cans are designed to allow controlled deformation, or "growth," of the can structure when environmental conditions cause the internal pressure to approach the maximum allowable pressure. This designed growth increases the internal volume of the can, causing a corresponding reduction in the interior pressure and thereby forestalling pressure-induced failure. In effect, this designed growth reduces the maximum internal pressure for given product fill volume, carbonation factor, and physical conditions, thus allowing the can's maximum allowable pressure to be lowered, and lightweighting efforts to progress.
However, in previous can designs allowing for can growth, the extent and location of the pressure-induced growth was highly dependent upon the specific design profile of the can bottom and the pressure history experienced by an individual can. This resulted in finished cans having variable dimensions in certain critical areas, adversely affecting the use of the can by the packager, shipper, retailer, and consumer.
A need exists, therefore, for a can having an ability for controlled growth such that maximum internal pressure is reduced for a given fill of product and physical conditions, and having finished dimensions that are only minimally dependent upon the pressure history of the individual can.
Among the external forces a can must withstand are axial loads imposed during filling and seaming operations. Conventional automated filling and seaming equipment presses down with great force on the upper rim of the can. The ability of the can to withstand these axial loads is termed "column strength." The supporting surfaces on the bottom of the can, which may comprise one or more annular surfaces or sets of discrete discontinuous surfaces, are typically called the bearing surfaces. This bearing surface is especially prone to failure during the filling and seaming operation, and this presents an obstacle to further lightweighting.
A need exists, therefore, for a controlled growth can having sufficient column strength to allow conventional filling and seaming operations.
Empty cans, especially if made of aluminum, are very light in weight. As a result, such cans are prone to topple from their upright position during processing in the brewery or canning plant, thereby causing increased can wastage and often disrupting operations. An important factor relating to the mobility of empty cans is the effective diameter of the bearing surface upon which an upright can rests, i.e., the diameter of a circle passing. through the bearing surface of the can bottom. This diameter is known as the stand diameter.
A need exists, therefore, for a controlled growth can having a large stand diameter when empty such that the empty can exhibits good stability during movement.
After filling the can body with product and sealing it by seaming on a lid, the overall weight of a can is greatly increased. Because of this increased weight, the primary factor affecting filled-can mobility is sliding friction between the bearing surface of the can and the work surfaces of equipment such as conveyers. Since bare aluminum is relatively soft and does not slide well on many surfaces, a friction-reducing "mobility coating" is commonly applied to the bearing surface of a can. While effective at reducing friction, mobility coatings degrade rapidly during processing due to abrasion. If the aluminum underlying the mobility coating is exposed by this degradation, friction increases significantly, as do associated operating problems.
A need exists, therefore, for a controlled growth can having a first bearing surface which is replaced with a second bearing surface during processing, where the second bearing surface was protected from abrasion while the first bearing surface was in use.
For the purposes of transportation, storage, and display, it is important that a filled, finished can be stackable, i.e., that the bottom surfaces of one can are precisely dimensioned to cooperate with the lid surfaces of a similar can directly below. Stackability is typically achieved by providing a can with a projecting bottom and a recessed lid such that the bottom of one such can fits precisely into or around the recessed lid of a similar can directly below but the bottom of the upper can does not touch the lid tab, rivet, or lid score features on the lid of the can below. In previous cans that allowed for can growth, the pressure-induced growth often produced unpredictable variations in the dimensions of can features critical for stackability, such as the annular rim on the bottom end wall. These variations had an undesirable effect on stackability.
A need exists, therefore, for a controlled growth can having predictable dimensions for can features critical to stackability, regardless of the pressure history of the individual can.
For purposes of product appearance, production handling, and ease of transportation, it is desirable to minimize variations in the finished overall height of a can. Many previous can designs used deformation of the bottom of the can to provide volumetric expansion to reduce internal pressure. Such cans often experienced height increases which were proportional to the maximum internal pressure experienced. Depending upon the design, such "growth" may or may not be reversible if the internal pressure is subsequently reduced. As a result of variations in filling, processing, handling, and other conditions, there may be considerable variation in the height of filled cans using previous can bottom designs.
A need exists, therefore, for a controlled growth can having a predictable overall package height after growth has occurred, regardless of conditions or the pressure history of the individual can.
For some cans using volumetric expansion to control internal pressure, the "expanded" structure of the can has a relatively wide, unsupported annular surface on the bottom between the bearing surface and the can side wall. Such an unsupported surface tends to flex repeatedly, especially when subjected to load and vibration during shipment and handling. This repeated flexing may result in fatigue cracking of either the can body material itself or one of the protective coatings applied to the interior or exterior surface of the can. In any case, such cracking is considered to be a failure of the can.
A need exists, therefore, for a controlled growth can having a bottom with only a narrow, relatively stiff annular section between the bearing surface and the can side wall.
While some products, such as traditional beers, are pasteurized or heat-treated after canning to eliminate pathogens, other products such as draft beers and carbonated soft drinks are produced using aseptic equipment or other facilities that do not require such heat treatment. Significantly higher internal pressures are generated in a can which is heat treated as compared to a can which is asepticly processed. It is desirable for manufacturers to produce a single can body design which can be used for all of these applications.
A need exists, therefore, for a controlled growth can having finished characteristics that are not dependent upon whether pasteurized, aseptic, or other production methods are used.
The detection of leaking cans under high-speed production conditions is another problem faced by can producers. In the case of minor leaks, the leak may not be readily apparent from the appearance of the can exterior. While radiation-based level detectors have been used, their performance for leak detection is subjective.
A need exists, therefore, for a controlled growth can having an external indication of leakage.
"Head space" refers to the partial can volume intentionally left empty of liquid during the filling process. In many cans, head space is provided in order to allow room for liquid expansion and for some of the dissolved CO.sub.2 in the liquid carbonated product to evolve into gas in the head space. However, head space can be a problem for two reasons. First, a large head space increases the chance that undesirable gases (also called "airs") will be introduced into the can during the filler/seamer transfer operation. These gases, primarily oxygen, tend to oxidize or otherwise degrade the product. Second, cans relying on head space alone to reduce the maximum internal pressure may experience over-pressuring if the can is overfilled during the filling operation, since this will necessarily cause the volume of the head space to be less than design specifications.
A need exists, therefore, for a controlled growth can having a decreased requirement for headspace during filling and a decreased sensitivity to overfilling.
3. Prior Art
The prior art contains many cans and containers, including those disclosed in U.S. Pat. Nos. 3,409,167, 3,904,069, 3,979,009, 4,037,752, 4,147,271, 4,222,494, 4,381,061, 4,412,627, 4,426,013, and 4,431,112. However, prior art cans typically focus on an improvement to only a single factor of can design, such as reduced maximum working pressure, rather than improvements to multiple factors.
For example, U.S. Pat. No. 3,979,009 to Walker discloses a bottom for a seamless metal container body wherein the central portion of the bottom includes a stiffening embossment that is joined to the other portions of the bottom by a hinge-like section that permits outward flexing or bulging of the bottom when the container is sealed and subjected to internal pressures. While this can may provide pressure reduction through volumetric expansion, reference to FIGS. 1 and 3 of the '009 patent reveals that the resulting bottom profile has very low stackability (i.e., if the can is stacked on a similar can, the bottom bearing surface, in either the original state or the "extended" state, will not fit within the rim of a similar lid so as to prevent lateral motion). Furthermore, as shown in FIG. 3, the can bottom in its "extended" shape has two wide, unsupported annular surfaces stretching outwardly from the primary annular stabilizing ring structure 26 to the third stabilizing ring structure 34. Such a wide unsupported annular surfaces are prone to cause repeated flexing and fatigue cracking of the can material or protective coatings.
Another example is U.S. Pat. No. 3,904,069 to Toukmanian, which discloses a metal cylindrical can body having a bottom wall structure that includes a centrally disposed circular depression 28 and which permits the can to expand in height, when subjected to internal pressure, by deforming into a shape in which the wide annular rim 26 of the depression 28 forms a base on which the can sits. While this can may provide pressure control through volumetric expansion, reference to paired FIGS. 1 and 5, and 2 and 6, respectively, of the '069 patent reveals that the resulting bottom profile of this can also has very low stackability. Furthermore, as shown in FIG. 11 of the '069 patent, the mobility of the filled can will be relatively low because the diameter of the bearing surface formed by the edge 30 of the depression 28 is small, and the mobility coating on the bearing surface is subject to continual degradation.
Yet other examples are U.S. Pat. Nos. 4,147,271 and 4,431,112 to Yamaguchi. These patents disclose variations of a drawn and ironed can body having a thinned bottom with a central portion which distends under internal pressure and an outer peripheral portion provided with buckling resistant strength sufficient to withstand the internal pressure. In the '271 patent, the central portion of the bottom is flat, as shown in FIGS. 10 and 14 of the '271 patent. In the '112 patent, the central portion is domed, as shown in FIG. 12 of the '112 patent. As with the Walker and Toukmanian, Yamaguchi thus provides pressure control through volumetric expansion. However, only the central portion of the bottom distends, as indicated by the dotted line in FIG. 14 of the '271 patent, and even in its distended form, this central portion remains above the end plane of the original can bottom. The outer peripheral portion of cans constructed according to Yamaguchi distends very little. Thus, the amount of volumetric expansion and pressure control achieved by Yamaguchi-type cans is small relative to cans in which the entire bottom wall extends. In addition, the stackability of cans constructed according to Yamaguchi may be impaired by the distension of the central portion of the bottom of one can into the area to be occupied by the lid of a second can stacked below. Further, the filled-can mobility of Yamaguchi-type cans will be impaired since only a single bearing surface, namely the outer peripheral portion of the bottom, is used despite its degradation during manufacture, production handling and transportation.